1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical recording medium drive employing the same.
2. Description of the Prior Art
An optical pickup apparatus employed for an optical recording medium drive such as an optical disk drive is adapted to record or read information in or from an optical recording medium such as an optical disk or detect a servo signal with a laser beam.
FIG. 20 schematically illustrates a conventional optical pickup apparatus disclosed in Japanese Patent Laying-Open Gazette No. 3-76035 (1991). This optical pickup apparatus performs tracking servo control by the three-beam method.
Referring to FIG. 20, symbols X, Y and Z denote the radial direction of an optical disk 1, the track direction of the optical disk 1, and a direction perpendicular to the disk plane of the optical disk 1 respectively.
A semiconductor laser device 102 emits a laser beam B in the direction Z. The beam B emitted from the semiconductor laser device 102 enters a diffraction grating 103. FIG. 21 is a plan view of the diffraction grating 103. The diffraction grating 103 has a grating surface 103a formed by unevenness of regular pitches. The grating surface 103a divides the incident laser beam B into three beams, i.e., a 0th order diffracted beam (main beam), a +1st order diffracted beam (subbeam) and a xe2x88x921st order diffracted beam (subbeam), and transmits the same through a transmission-type holographic optical element 104.
Referring to FIG. 20, an objective lens 105 condenses the three beams transmitted through the transmission-type holographic optical element 104 on the optical disk 1. FIG. 22 is a model diagram showing the condensed states on the recording plane of the optical disk 1. As shown in FIG. 22, the 0th order diffracted beam is condensed on a track surface TR of the recording plane as a main spot M0, and the xc2x11st order diffracted beams are condensed on both sides of the main spot MO as subspots S1 and S2 respectively.
The transmission-type holographic optical element 104 diffracts three returned beams (reflected beams) from the main spot MO and the subspots S1 and S2 in a plane substantially including the directions X and Z, so that a photodetector 106 detects these returned beams.
FIG. 23 is a typical plan view showing an exemplary photodetector 106. This photodetector 106 includes a photodetection part 106a provided on the central portion for performing focus servo control with the astigmatism method and photodetection parts 106b and 106c provided on both sides of the photodetection part 106a for performing tracking servo control with the three-beam method. The returned beam corresponding to the main spot M0 enters the central portion of the photodetection part 106a while the returned beams corresponding to the subspots S1 and S2 enter the photodetection parts 106b and 106c respectively.
The aforementioned optical pickup apparatus performs tracking control in the following manner: As shown in FIG. 22, the track surface TR recording information is different in light reflectance from a non-track surface. When the photodetection parts 106b and 106c detect the returned beams from the subspots S1 and S2, the returned beams from the two subspots S1 and S2 entering the two photodetection parts 106b and 106c are equal in light intensity to each other if the main spot MO excellently tracks the track surface TR to be reproduced. If the main spot MO deviates to either side of the track surface TR, on the other hand, the photodetection part 106a or 106b relatively largely detects the light intensity of the returned beam from one of the subspots S1 and S2. With output signals E and F from the photodetection parts 106b and 106c, therefore, the following tracking error signal TE is obtained:
xe2x80x83TE=Exe2x88x92F
The optical pickup apparatus performs excellent tracking control when the tracking error signal TE is zero, and detects deterioration of the tracking state as the value of the tracking error signal TE increases.
When detecting the tracking error signal TE, the optical pickup apparatus moves the objective lens 105 in the radial direction (the direction X), for correcting the condensed positions of the main spot M0 and the subspots S1 and S2 on the track surface TR of the optical disk 1.
FIG. 24A is a typical sectional view showing the condensed states of diffracted beams B1 and B2 diffracted by the diffraction grating 103, and FIG. 24B shows typical plan views of the objective lens 105. As shown in FIG. 24A, the diffracted beam B1 diffracted by the diffraction grating 103 in the +1st order direction passes through the objective lens 105, to be condensed as the subspot S1. The diffracted beam B2 diffracted in the xe2x88x921st order direction passes through the objective lens 105, to be condensed as the subspot S2.
Referring to FIG. 24B, the grating surface 103a of the diffraction grating 103 is formed to be larger than the laser beam B, as shown in FIG. 20. Therefore, the laser beam B incident on the grating surface 103a is diffracted over a region wider than an aperture 105a of the objective lens 105, to result in regions B1a and B2a not entering the aperture 105a of the objective lens 105.
When the optical pickup apparatus performs a tracking operation in this state and moves the objective lens 105 in the direction X (the radial direction of the optical disk 1), the incident states of the diffracted beams B1 and B2 on the objective lens 105 change from those on the left to those on the right in FIG. 24B. The ratios of the diffracted beams B1 and B2 entering the aperture 105a of the objective lens 105 reduce following movement of the objective lens 105. Therefore, the light quantities of the subspots S1 and S2 reduce on the recording plane 1a of the optical disk 1, to result in reduction of the light quantities of the returned beams from the subspots S1 and S2 entering the photodetection parts 106b and 106c. When the objective lens 105 is moved during the tracking operation, therefore, the output of the tracking error signal TE disadvantageously reduces.
FIG. 25 is a model diagram for illustrating the diffracted state of the beam B diffracted by the diffraction grating 105. Referring to FIG. 25, a light source 200 forms an emissive end of the semiconductor laser device 102, so that the laser beam B emitted from this light source 200 is condensed on the recording plane 1a of the optical disk 1 as the two subspots S1 and S2. The transmission-type holographic optical element 104 is omitted in FIG. 25.
The grating surface 103a diffracts the laser beam B emitted from the light source 200 at least in the +1st order direction and the xe2x88x921st order direction. In the laser beam B, the +1st order diffracted partial beam of a partial beam BE1 passes through the objective lens 105, to be condensed as the subspot S1. The +1st order diffracted partial beam of a partial beam BE2 passes through a part beyond the objective lens 105, not to be condensed on the subspot S1.
On the other hand, the xe2x88x921st order diffracted partial beam of a partial beam BE3 passes through the objective lens 105, to be condensed on the subspot S2. Further, the xe2x88x921st order diffracted partial beam of a partial beam BE4 passes through a part beyond the objective lens 105, not to be condensed on the subspot S2.
When an optical axis LP passing through the peak of the light intensity distribution of the laser beam B aligns with a central axis Z0 passing through the center of the objective lens 105, the light quantities of the partial beams BE1 and BE3 condensed on the subspots S1 and S2 respectively are equal to each other. Therefore, the correct tracking state can be detected by detecting the difference between the light quantities of the returned beams from the two subspots S1 and S2.
However, the optical axis LP of the laser beam B may deviate from the central axis Z0 of the objective lens 105 due to a locational error of the semiconductor laser device 102 or the emission property of the laser beam B. When the optical axis LP deviates from the central axis Z0, the partial beams BE1 and BE3 are condensed on the two subspots S1 and S2 in non-uniform light quantities.
FIGS. 26A and 26B illustrate light intensity distribution states of the laser beam B in a section taken along the line Qxe2x80x94Q in FIG. 25. In FIGS. 26A and 26B, a symbol 2R denotes the diameter of the partial beam incidenting into the objective lens 105 within the +1st and the xe2x88x921st order diffracted beams. The optical axis LP aligns with the central axis Z0 in FIG. 26A, while the former deviates from the latter in FIG. 26B. FIG. 26A shows the light quantities corresponding to the partial beams BE1 and BE2 in regions (E1+E2) and (E3) respectively. Further, the light quantities corresponding to the partial beams BE3 and BE4 are shown in regions (E1+E3) and (E2) respectively.
As shown in FIG. 26A, the light quantity (the region (E1+E2)) of the partial beam BE1 condensed on the subspot S1 is equal to the light quantity (the region (E1+E3)) of the partial beam BE3 condensed on the subspot S2 when the optical axis LP aligns with the central axis Z0.
When the optical axis LP deviates from the central axis Z0, on the other hand, the light quantities of the partial beams BE1 and BE3 condensed on the subspots S1 and S2, which are shown in regions (E1+E20) and (E1+E30) respectively, differ from each other. Thus, the tracking error signal TES based on the returned beams from the two subspots S1 and S2 is so offset that it is difficult to detect the correct tracking state.
FIG. 27 schematically illustrates another conventional optical pickup apparatus. This optical pickup apparatus is adapted to perform tracking servo control and focus servo control with the three-beam method and the astigmatism method respectively.
Referring to FIG. 27, a laser beam 112 emitted from a semiconductor laser device 121 passes through a transmission-type diffraction grating 123 to be divided into three beams, i.e., a 0th order diffracted beam (main beam) and xc2x11st order diffracted beams (subbeams) and transmitted through a transmission-type holographic optical element 124.
An objective lens 116 condenses the three beams transmitted through the transmission-type holographic optical element 124 on an optical disk 1 as a main spot M0 and subspots S1 and S2 located on both sides thereof. An actuator 140 supports the objective lens 116 to be movable in the radial direction (the X-axis direction) of the optical disk 1 for a tracking operation and to be movable in the Y-axis direction for a focus operation.
FIG. 28 illustrates the main spot M0 and the subspots S1 and S2 formed on the optical disk 1. As shown in FIG. 28, the optical system of the optical pickup apparatus is so adjusted that the main spot M0 scans a track TR to be reproduced and the subspots S1 and S2 scan both sides of the main spot M0 slightly over the track TR.
The transmission-type holographic optical element 124 diffracts three returned beams (reflected beams) from the optical disk 1, so that a signal detection photodiode 133 detects the same.
FIG. 29 is a typical plan view showing an exemplary signal detection photodiode 133. This signal detection photodiode 133 includes photodetection parts 150a to 150d provided on the central portion for performing focus servo control with the astigmatism method and photodetection parts 150e and 150f provided on both sides of the photodetection parts 150a to 150d for performing tracking servo control with the three-beam method. The returned beam (main beam) corresponding to the main spot M0 enters the central portion of the photodetection parts 150a to 150d, while returned beams (subspots) 112a and 112b corresponding to the subspots S1 and S2 enter the photodetection parts 150e and 150f respectively.
On the basis of detection signals E and F from the photodetection parts 150e and 150f of the signal detection photodiode 133 receiving the returned beams (subbeams) 112a and 112b, the optical pickup apparatus performs the tracking operation in the following manner:
FIG. 30 is a circuit diagram showing respective parts of an optical disk drive comprising the optical pickup apparatus 100 performing the tracking operation. Referring to FIG. 30, the photodetection parts 150e and 150f of the signal detection photodiode 133 of the optical pickup apparatus 100 output the detection signals E and F to an E-F processing part 155 provided on a driving circuit part 154 of the optical disk drive. With the detection signals E and F received from the photodetection parts 150e and 150f, the E-F processing part 155 obtains the following tracking error signal TE:
TE=Exe2x88x92F
The tracking error signal TE is inputted in an operational amplifier 158 of a servo circuit 157 through a low-pass filter 156, amplified and thereafter supplied to a tracking coil 142 of the actuator 140 of the optical pickup apparatus 100.
As shown in FIG. 27, the actuator 140 supports the objective lens 116 to be movable in the radial direction (the X-axis direction) of the optical disk 1. The actuator 140 comprises a holder 141 for holding the objective lens 116, the tracking coil 142 connected to the holder 141 to be movable in the radial direction, and a permanent magnet 144 separating from the tracking coil 142. When a driving voltage is applied to the tracking coil 142, the actuator 140 moves the objective lens 116 in the X-axis direction by electromagnetic force caused between the tracking coil 142 and the permanent magnet 144.
When the main spot M0 formed on the optical disk 1 effectively tracks the track TR to be reproduced in FIG. 28, the returned beams 112a and 112b from the two subspots S1 and S2 enter the photodetection parts 150e and 150f in equal light intensity. Therefore, the tracking error signal TE outputted from the E-F processing part 155 is zero and no driving voltage is applied to the tracking coil 142 of the actuator 140. Thus, the objective lens 116 maintains its state.
When the main spot MO deviates to either side of the track TR to be reproduced, on the other hand, the light intensity of the returned beam 112a or 112b from the subspot S1 or S2 increases. Thus, the detection signals E and F from the photodetection parts 150e and 150f differ from each other. Therefore, the E-F processing part 155 outputs the tracking error signal TE, which in turn is amplified by the operational amplifier 158 of the servo circuit 157 so that a driving voltage is applied to the tracking coil 142 and the actuator 140 radially moves the objective lens 116 for correcting the position of the main spot M1.
In recent years, miniaturization of such an optical pickup apparatus 100 is strongly desired, and the respective elements thereof are miniaturized with reduction of the diameter of the objective lens 116. In an assembling step for the optical pickup apparatus 100, therefore, it is difficult to correctly align the objective lens 116 with the optical path of the laser beam 112.
FIG. 31 is a typical plan view showing an incident state of the laser beam 112 on the objective lens 116. In the optical pickup apparatus 100, the semiconductor laser device 121, the diffraction grating 123 and the transmission-type holographic optical element 124 are integrated into a unit independently of the objective lens 116, and these units are assembled with each other in alignment. In assembling, therefore, the optical axis of the objective lens 116 may deviate from those of the two subbeams 112a and 112b of the laser beam 112 by d along the radial direction (the X-axis direction) of the optical disk 1, as shown in FIG. 31.
Such deviation d in the mounting position of the objective lens 116 results in the following disadvantage: The optical disk drive moves the objective lens 116 by a constant distance in the radial direction of the optical disk 1 in order to search the program for a tune recorded in the optical disk 1, for example. If the optical axis of the objective lens 116 deviates from those of the subbeams 112a and 112b of the laser beam 112 by d in assembling as shown in FIG. 31, however, the subbeams 112a and 112b pass through the objective lens 116 in different light quantities following movement of the objective lens 116 for the program search, in response to the direction of movement. The light quantities of the subbeams 112a and 112b passing through the objective lens 116 extremely reduce following movement of the objective lens 116 in one direction, and hence the output of the tracking error signal TE based on the subbeams 112a and 112b passing through the objective lens 116 reduces to hinder a effective tracking operation.
FIG. 32 illustrates changes of the tracking error signal TE following movement of the objective lens 116. Referring to FIG. 32, the horizontal axis shows the direction and the distance of movement of the objective lens 116, and the vertical axis shows the tracking error signal TE. When the center of the objective lens 116 aligns with the optical axis of the laser beam 112 in the radial direction of the optical disk 1, symmetrical distribution TE0 of the tracking error signal TE is obtained following movement of the objective lens 116, as shown by a dotted line in FIG. 32. When the center of the objective lens 116 deviates from the optical axis of the laser beam 112, on the other hand, asymmetrical distribution TE1 of the tracking error signal TE is obtained depending on the direction of movement of the objective lens 116, as shown by a solid line. The tracking error signal TE reduces below an output value A necessary for tracking on a position of movement of the objective lens 116, to hinder correct program search.
In general, therefore, an offset circuit 159 is provided on one input side of the operational amplifier 158 of the servo circuit 157, in order to correct the deviation of the objective lens 116 from the optical axis of the laser beam 112. In the optical pickup apparatus 100 built into the optical disk drive, the offset circuit 159 corrects the deviation of the objective lens 116 as follows:
The offset circuit 159 moves the objective lens 116 along the radial direction toward the center and the outer periphery respectively by prescribed distances of 400 xcexcm, for example, and detects the voltages of the tracking error signal TE. If the center of the objective lens 116 deviates from the optical axis of the laser beam 112, the tracking error signal TE1 exhibits different voltages following movement of the objective lens 116 toward the center and the outer periphery, as shown in FIG. 32. Therefore, the movement origin position (the position of the objective lens 116 performing no tracking operation) is moved for equalizing the voltages of the tracking error signal TE in movement of the objective lens 116 toward the center and the outer periphery.
The resistance value of a variable resistor 160 of the offset circuit 159 is adjusted and a driving voltage is applied to the tracking coil 142 for moving the movement origin position of the objective lens 116 in the radial direction of the optical disk 1. Further, the objective lens 116 is moved from the movement origin position along the radial direction of the optical disk 1 toward the center and the outer periphery by prescribed distances respectively, for detecting the current values of the tracking error signal TE. Adjustment of the variable resistor 160 of the offset circuit 159 is ended when the detected values of the tracking error signal TE are equal to each other in movement toward the center and the outer periphery. Thus, the deviation of the objective lens 116 from the optical axis of the laser beam 112 in the radial direction of the optical disk 1 can be corrected.
However, the optical pickup apparatus 100 may be independently manufactured and put on the market by a manufacturer different from that for the optical disk drive employing the same. In this case, therefore, the manufacturer for the optical disk drive or the like must adjust the deviation of the objective lens 116 of the optical pickup apparatus 100 with complicated assembling and adjusting operations.
An object of the present invention is to provide an optical pickup apparatus causing no output reduction of a tracking signal following movement of an objective lens in a tracking operation and an optical recording medium drive employing the same.
Another object of the present invention is to provide an optical pickup apparatus capable of suppressing offset of a tracking error signal resulting from optical axis deviation of a beam emitted from a light source and an optical recording medium drive employing the same.
Still another object of the present invention is to provide an optical pickup apparatus capable of adjusting the position of an objective lens with respect to the optical axis of a laser beam in manufacturing, an optical recording medium drive comprising the same, and a method of adjusting an optical pickup apparatus.
The optical pickup apparatus according to the present invention comprises a light source for emitting a beam, a first diffraction element having a diffraction surface for diffracting the beam emitted from the light source at least in first and second directions, and an objective lens for irradiating an optical recording medium with beams diffracted by the first diffraction element in the first and second directions respectively. The objective lens is provided to be movable along the radial direction of the optical recording medium for a tracking operation, and the diffraction surface of the first diffraction element is formed in dimensions for locating a light spot formed on the objective lens by the diffracted beams diffracted by the diffraction surface in the first and second directions respectively in an aperture of the objective lens following movement of the objective lens for the tracking operation.
Also when the objective lens is moved for the tracking operation, all diffracted beams pass through the objective lens, to be condensed on the optical recording medium. Therefore, the light quantities of the diffracted beams condensed on the optical recording medium remain unchanged regardless of movement of the objective lens. Thus, it is possible to prevent the output of a tracking error signal from fluctuation resulting from change of the light quantities of the diffracted beams on the optical recording medium resulting from to the tracking operation.
In particular, the diffraction surface of the first diffraction element is preferably formed in a rectangular shape smaller than a light spot formed on the first diffraction element by the beam emitted from the light source in dimensions for locating a rectangular light spot formed on the objective lens by the diffracted beams diffracted by the diffraction surface in the first and second directions respectively following movement of the objective lens in the aperture of the objective lens.
In this case, the rectangular light spot of the diffracted beams enters the aperture of the objective lens regardless of the movement of the objective lens. Therefore, the light quantities of the diffracted beams condensed on the optical recording medium are maintained constant also when the objective lens is moved, whereby the tracking error signal based on the diffracted beams can be prevented from fluctuation.
Particularly assuming that R and Q represent the aperture radius and the amount of movement of the objective lens respectively, L1 and L2 represent effective distances between the light source and the center of the objective lens and between the diffraction surface and the light source respectively, S represents the distance between a first virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the first direction toward the objective lens and the light source or between a second virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the second direction toward the objective lens and the light source, and B1 represents a limit value for the rectangular light spot formed on the objective lens in a direction perpendicular to the direction of movement of the objective lens, the width W1 of the diffraction surface of the first diffraction element in the direction perpendicular to the direction of movement of the objective lens is preferably set to satisfy:
W1 less than 2xc3x97{{square root over (R2xe2x88x92Q2)}xe2x88x92S}xc3x97L2/L1+2S
and the width W2 of the diffraction surface in the direction of movement of the objective lens is preferably set to satisfy:
W2xe2x89xa6{{square root over ((2R)2xe2x88x92(B1)2)}xe2x88x922Q}xc3x97L2/L1
with the following limit value B1:
B1=(W1xe2x88x922S)xc3x97L1/L2+2S
The diffraction surface satisfying the above conditions introduces all diffracted beams diffracted in the first and second directions into the aperture of the objective lens regardless of movement of the objective lens. Thus, the light quantities of the diffracted beams are prevented from variation with movement of the objective lens.
In particular, the optical pickup apparatus provided with the first diffraction element having the diffraction surface formed in a rectangular shape may further comprise a second diffraction element for transmitting the beams diffracted by the first diffraction element in the first and second directions respectively and guiding the same to the objective lens while diffracting returned beams from the optical recording medium, and a photodetector for receiving the returned beams diffracted by the second diffraction element.
In this case, the second diffraction element diffracts the returned beams, so that the returned beams from the optical recording medium can be guided to the photodetector, which in turn detects the tracking error signal.
The diffract ion surface of the first diffraction element is preferably formed in an elliptic or circular shape smaller than the light spot formed on the first diffraction element by the beam emitted from the light source in dimensions for locating an elliptic light spot formed on the objective lens by the diffracted beams diffracted by the diffraction surface in the first and second directions respectively in the aperture of the objective lens following movement of the objective lens.
In this case, all elliptic or circular diffracted beams can be introduced into the aperture of the objective lens regardless of movement of the objective lens in the tracking operation.
Particularly assuming that the elliptic diffraction surface of the first diffraction element has its major axis in the direction perpendicular to the direction of movement of the objective lens, R and Q represent the aperture radius and the amount of movement of the objective lens respectively, L1 and L2 represent effective distances between the light source and the center of the objective lens and between the diffraction surface and the light source respectively, S represents the distance between a first virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the first direction toward the objective lens and the light source or between a second virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the second direction toward the objective lens and the light source, b represents a limit value for the radius of the elliptic light spot formed on the objective lens in the direction perpendicular to the direction of movement of the objective lens and WB represents the width of the elliptic diffraction surface in the direction perpendicular to the direction of movement of the objective lens, the width WA of the elliptic diffraction surface in the direction of movement of the objective lens is preferably set to satisfy:
WAxe2x89xa62xc3x97{{square root over (b2Q2/(b2xe2x88x92R2)+b2)}}xc3x97L2/L1
where
b=(WBxe2x88x922S)xc3x97L1/L2+2S
Further, the width WB of the elliptic diffraction surface of the first diffraction element in the direction perpendicular to the direction of movement of the objective lens is preferably set to satisfy:
2xc3x97[L2/L1xc3x97{{square root over (Rxc3x97(Rxe2x88x92Q))}xe2x88x92S}+S]xe2x89xa6WB less than 2xc3x97[L2/L1xc3x97{{square root over (R2xe2x88x92Q2)}xe2x88x92S}+S]
Assuming that R and Q represent the aperture radius and the amount of movement of the objective lens respectively and L1 and L2 represent effective distances between the light source and the center of the objective lens and between the diffraction surface and the light source respectively, in addition, the width WA of the elliptic diffraction surface in the direction of movement of the objective lens is preferably set to satisfy:
WAxe2x89xa62xc3x97(Rxe2x88x92Q)xc3x97L2/L1
Assuming that S represents the distance between the first virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the first direction toward the objective lens and the light source or between the second virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the second direction toward the objective lens and the light source, further, the width WB of the elliptic diffraction surface of the first diffraction element in the direction perpendicular to the direction of movement of the objective lens is preferably set to satisfy:
WB less than 2xc3x97[L2/L1xc3x97{{square root over (Rxc3x97(Rxe2x88x92Q))}xe2x88x92S}+S]
In particular, the optical pickup apparatus having the first diffraction element having the elliptically formed diffraction surface may further comprise a second diffraction element for transmitting the beams diffracted by the first diffraction element in the first and second directions respectively, guiding the same to the objective lens and diffracting the returned beams from the optical recording medium, and a photodetector for receiving the returned beams diffracted by the second diffraction element.
In this case, the second diffraction element diffracts the returned beams from the optical recording medium for guiding the same to the photodetector, so that the tracking error signal by the photodetector can be detected.
An optical pickup apparatus according to another aspect of the present invention comprises a light source for emitting a beam, a first diffraction element having a diffraction surface for diffracting the beam emitted from the light source at least in first and second directions, and an objective lens for irradiating an optical recording medium with beams diffracted by the first diffraction element in the first and second directions respectively. The width of the first diffraction element in a plane including the optical axis of the beam emitted from the light source and axes of the beams diffracted in the first and second directions respectively is set to be smaller than that of a region including a first light spot and a second light spot. The first light spot is a light spot on the first diffraction element corresponding to a part of the beam, diffracted by the first diffraction element in the first direction, entering the objective lens in the beam emitted from the light source. The second light spot is a light spot on the first diffraction element corresponding to a part of the beam, diffracted by the first diffraction element in the second direction, entering the objective lens in the beam emitted from the light source.
The beam emitted from the light source includes the beam diffracted by the diffraction surface of the first diffraction element only in the first direction to enter the objective lens, a beam diffracted only in the second direction to enter the objective lens, and a beam diffracted in the first and second directions to enter the objective lens. The width of the diffraction surface of the first diffraction element is rendered smaller than that of the region including the first and second light spots formed on the first diffraction element, whereby the part corresponding to the beam diffracted only in the first direction to enter the objective lens and that corresponding to the beam diffracted only in the second direction to enter the objective lens can be reduced in a region incident on the diffraction surface. Thus, it is possible to inhibit the light quantities of the beams diffracted in the first and second directions from non-uniformity resulting from optical axis deviation of the optical axis of the beam emitted from the light source, thereby suppressing non-uniform output of a tracking error detection signal utilizing the beams diffracted in the first and second directions respectively following optical axis deviation.
In particular, the width of the diffraction surface of the first diffraction element in the aforementioned plane is preferably set to be smaller than that of an overlap region of the first and second light spots on the first diffraction element.
In this case, the light quantities of the diffracted beams diffracted in the first and second directions respectively to enter the objective lens are equally changed even if the optical axis of the beam emitted from the light source deviates from a prescribed direction. Thus, the tracking error signal based on the diffracted beams in the first and second directions is reliably prevented from offset.
In particular, the first and second directions for diffracting the beams by the diffraction surface of the first diffraction element are preferably +1st and xe2x88x921st order directions respectively.
Assuming that R represents the aperture radius of the objective lens, L1 and L2 represent effective distances between the light source and the center of the objective lens and between the diffraction surface and the light source respectively and S represents the distance between a first virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the first direction toward the objective lens and the light source or between a second virtual light source supposed to emit a straight beam equivalent to the beam diffracted in the second direction toward the objective lens and the light source, the width W of the diffraction surface of the first diffraction element is preferably set to satisfy the following relation:
Wxe2x89xa62xc3x97{(R+S)xc3x97L2/L1xe2x88x92S}
When the width W of the diffraction surface of the first diffraction element is set to satisfy the above relation, the beams of the beam emitted from the light source diffracted in the first and second directions, based on a common beam part, enter the objective lens. Even if the optical axis of the beam emitted from the light source deviates, therefore, the light quantities of the diffracted beams diffracted in the first and second directions are equally changed. Thus, it is possible to prevent offset of the tracking error signal based on the beams diffracted in the first and second directions.
Assuming that X1 represents the physical distance between the light source and the center of the objective lens and d and n represent the thickness and the refractive index of the first diffraction element respectively, the effective distance L1 is defined as follows:
L1=X1xe2x88x92(nxe2x88x921)xc3x97d/n
Assuming that X2 represents the physical distance between the light source and the diffraction surface and d and n represent the thickness and the refractive index of the first diffraction element respectively, the effective distance L2 is defined as follows:
L2=X2xe2x88x92(nxe2x88x921)xc3x97d/n
An optical pickup apparatus according to still another aspect of the present invention comprises a light source emitting a beam, a first diffraction element for diffracting the beam emitted from the light source at least in first and second directions, and an objective lens for irradiating an optical recording medium with beams diffracted by the first diffraction element in the first and second directions respectively. The objective lens is provided to be movable along the radial direction of the optical recording medium for a tracking operation, and the diffraction surface of the first diffraction element is so formed that the width in a plane including the optical axis of the beam emitted from the light source and axes of the beams diffracted in the first and second directions respectively is smaller than that of a region including a first light spot and a second light spot and dimensions are set for locating a light spot formed on the objective lens by the beams diffracted by the diffraction surface in the first and second directions respectively in an aperture of the objective lens following movement of the objective lens for the tracking operation. The first light spot is a light spot on the first diffraction element corresponding to a part of the beam, diffracted by the first diffraction element in the first direction, entering the objective lens in the beam emitted from the light source and the second light spot is a light spot on the first diffraction element corresponding to a part of the beam, diffracted by the first diffraction element in the second direction, entering the objective lens in the beam emitted from the light source.
In this case, the width of the diffraction surface of the first diffraction element is rendered smaller than that of the region including the first and second light spots formed on the first diffraction element thereby reducing non-uniformity of the light quantities of the diffracted beams in the first and second directions resulting from optical axis deviation of the beam emitted from the light source, to be capable of suppressing non-uniform output of a tracking error detection signal utilizing the diffracted beams diffracted in the first and second directions resulting from optical axis deviation.
Even if the objective lens is moved for the tracking operation, all diffracted beams pass through the objective lens to be condensed on the optical recording medium. Therefore, the light quantities of the diffracted beams condensed on the optical recording medium remain unchanged regardless of movement of the objective lens. Thus, it is possible to prevent output fluctuation of the tracking error signal resulting from change of the light quantities of the diffracted beams on the optical recording medium following the tracking operation.
In particular, the width of the diffraction surface of the first diffraction element in the aforementioned plane is preferably set to be smaller than that of an overlap region of the first and second light spots formed on the first diffraction element.
In this case, the light quantities of the beams diffracted in the first and second directions to enter the objective lens are equally changed even if the optical axis of the beam emitted from the light source deviates from a prescribed direction. Thus, it is possible to reliably prevent offset of the tracking error signal based on the diffracted beams in the first and second directions.
The optical pickup apparatus may further comprise a second diffraction element for guiding the beams diffracted by the first diffraction element in the first and second directions respectively and guiding the same to the objective lens while diffracting returned beams from the optical recording medium and a photodetector for receiving the returned beams diffracted by the second diffraction element.
In this case, the second diffraction element diffracts the returned beams from the optical recording medium for guiding the same to the photodetector, so that the tracking error signal by the photodetector can be detected.
An optical pickup apparatus according to a further aspect of the present invention, which can detect a tracking state of a beam for reading information from an optical recording medium, comprises a light source for emitting the beam, a first diffraction element for dividing the beam emitted from the light source into a plurality of beams for tracking state detection, an objective lens provided to be movable in the radial direction of the optical recording medium for condensing the plurality of beams divided by the first diffraction element on the optical recording medium, a photodetector having a plurality of photodetection parts for receiving a plurality of returned beams based on the plurality of beams for tracking state detection respectively and outputting a plurality of detection signals responsive to the received light quantities, an adjusting circuit capable of changing the plurality of detection signals outputted from the plurality of photoreceiving parts of the photodetector, and a lens driving part for radially moving the objective lens in response to a prescribed signal based on the plurality of detection signals adjusted by the adjusting circuit.
The optical pickup apparatus according to this aspect of the present invention can change the plurality of detection signals for tracking state detection by the adjusting circuit for moving the objective lens by the prescribed signal based on the changed detection signals. Thus, it is possible to correct deviation of the objective lens by adjusting the adjusting circuit for changing the detection signals in correspondence to the amount of deviation of the central portion of the objective lens from the optical axis of the beam.
Further, the optical pickup apparatus itself is provided with the adjusting circuit, whereby the adjusting circuit can be adjusted in an assembling stage of the optical pickup apparatus. Thus, adjustment for correcting deviation of the objective lens can be omitted in an apparatus assembled with the optical pickup apparatus.
In particular, the adjusting circuit preferably includes a variable resistor for changing the plurality of detection signals outputted from the plurality of photoreceiving parts of the photodetector.
In this case, the resistance value of the variable resistor can be so adjusted as to readily change the prescribed signal supplied to the lens driving part, for correcting deviation of the objective lens.
In particular, the optical pickup apparatus may further comprise a wiring part for extracting the signals from the plurality of photoreceiving parts of the photodetector, so that the variable resistor is arranged on the wiring part.
In this case, it is possible to readily change the prescribed signal supplied to the lens driving part by adjusting the resistance value of the variable resistor arranged on the wiring part, for correcting deviation of the objective lens.
In particular, the wiring part is preferably formed on a flexible circuit board. In this case, the degree of freedom in mounting of the wiring part in the optical pickup apparatus is improved due to the flexibility of the flexible circuit board, so that the optical pickup apparatus can be minimized.
In particular, the optical pickup apparatus preferably further comprises a plurality of amplifier parts provided in correspondence to the plurality of photoreceiving parts in the photodetector for amplifying differences between the detection signals outputted from the corresponding photoreceiving parts and a reference signal respectively, and the adjusting circuit preferably includes a variable resistor for changing the reference signal supplied to at least one of the plurality of amplifier parts.
In this case, it is possible to readily change the prescribed signal supplied to the lens driving part by adjusting the variable resistor and changing the reference signal, for correcting deviation of the objective lens.
In particular, the photoreceiving parts and the plurality of amplifier parts are formed on a single chip. In this case, the optical pickup apparatus is suitable for miniaturization.
The optical recording medium drive according to the present invention, which is adapted to optically read information from an optical recording medium, comprises a rotation driving part for rotating the optical recording medium, an optical pickup apparatus for irradiating the optical recording medium with a laser beam and receiving a returned beam from the optical recording medium, a pickup driving part for moving the optical pickup apparatus in the radial direction of the optical recording medium, and a signal processing part for processing an output signal from the optical pickup apparatus. Further, the optical pickup apparatus comprises a light source for emitting the beam, a diffraction element having a diffraction surface for diffracting the beam emitted from the light source at least in first and second directions, and an objective lens for irradiating the optical recording medium with beams diffracted by the diffraction element in the first and second directions respectively. The objective lens is provided to be movable along the radial direction of the optical recording medium for a tracking operation, and the diffraction surface of the diffraction element is formed in dimensions for locating a light spot formed on the objective lens by the beams diffracted by the diffraction surface in the first and second directions respectively in an aperture of the objective lens following movement of the objective lens for the tracking operation.
Thus, the light quantities of the diffracted beams on the optical recording medium remain unchanged in the tracking operation, and an optical recording medium drive causing no output reduction of a tracking error signal can be obtained.
An optical recording medium drive unit according to a further aspect of the present invention, which is adapted to optically read information from an optical recording medium, comprises a rotation driving part for rotating the optical recording medium, an optical pickup apparatus for irradiating the optical recording medium with a laser beam and receiving a returned beam from the optical recording medium, a pickup driving part for moving the optical pickup apparatus in the radial direction of the optical recording medium, and a signal processing part for processing an output signal from the optical pickup apparatus. Further, the optical pickup apparatus comprises a light source for emitting the beam, a diffraction element having a diffraction surface for diffracting the beam emitted from the light source at least in first and second directions, and an objective lens for irradiating the optical recording medium with beams diffracted by the diffraction element in the first and second directions respectively. The width of the diffraction surface of the diffraction element in a plane including the optical axis of the beam emitted from the light source and axes of the beams diffracted in the first and second directions is set to be smaller than the width of a region including a first light spot and a second light spot. The first light spot is a light spot on the diffraction element corresponding to a part of the beam, diffracted by the diffraction element in the first direction, entering the objective lens in the beam emitted from the light source and the second light spot is a light spot on the diffraction element corresponding to a part of the beam, diffracted by the diffraction element in the second direction, entering the objective lens in the beam emitted from the light source.
In this case, offset of a tracking error signal is prevented even if the optical axis of the beam from the light source in the optical pickup apparatus deviates, whereby no offset adjustment is required and correct tracking control can be performed.
An optical recording medium drive according to a further aspect of the present invention, which is adapted to optically read information from an optical recording medium, comprises a rotation driving part for rotating the optical recording medium, an optical pickup apparatus for irradiating the optical recording medium with a laser beam and receiving a returned beam from the optical recording medium, a pickup driving part for moving the optical pickup apparatus in the radial direction of the optical recording medium, and a signal processing part for processing an output signal from the optical pickup apparatus. Further, the optical pickup apparatus comprises a light source for outputting the beam, a diffraction element for dividing the beam emitted from the light source into a plurality of beams for tracking state detection, an objective lens provided to be movable in the radial direction of the optical recording medium for condensing the plurality of beams divided by the diffraction element on the optical recording medium, a photodetector having a plurality of photoreceiving parts for receiving a plurality of returned beams based on the plurality of beams for tracking state detection condensed on the optical recording medium respectively for outputting detection signals responsive to the received light quantities, an adjusting circuit capable of changing the plurality of detection signals outputted from the plurality of photoreceiving parts of the photodetector, and a lens driving part for radially moving the objective lens in response to a prescribed signal based on the plurality of detection signals adjusted by the adjusting circuit.
The optical pickup apparatus adjusted to output a tracking signal for correcting deviation of the objective lens is assembled into the optical recording medium drive according to the present invention, whereby no adjustment of deviation of the objective lens of the optical pickup apparatus is required after assembling and the optical recording medium drive is easy to assemble.
The method of adjusting an optical pickup apparatus according to the present invention, which is adapted to correct deviation of a central portion of an objective lens with respect to the optical axes of a plurality of beams in the radial direction of an optical recording medium in the optical pickup apparatus comprising a light source for emitting a beam, a diffraction element for dividing the beam emitted from the light source into the plurality of beams for tracking state detection, the objective lens for condensing the plurality of beams divided by the diffraction element on the optical recording medium, a lens driving part for moving the objective lens in the radial direction of the optical recording medium, and a photodetector having a plurality of photoreceiving parts for receiving a plurality of returned beams based on the plurality of beams for tracking state detection condensed on the optical recording medium respectively and outputting a plurality of detection signals responsive to the received light quantities, provides the optical pickup apparatus with an adjusting circuit capable of changing the detection signals outputted from the plurality of photoreceiving parts, connects a driving circuit for generating a driving signal for moving the objective lens in the radial direction on the basis of the detection signals outputted from the photodetector through the adjusting circuit to the lens driving part of the optical pickup apparatus, moves the objective lens in the radial direction by changing the detection signals with the adjusting circuit and thereafter observes change of the detection signals while radially moving the objective lens by a prescribed distance, thereby correcting deviation of the central portion of the objective lens with respect to the optical axes of the plurality of beams in the radial direction of the optical recording medium.
The method of adjusting an optical pickup apparatus according to the present invention connects the previously prepared driving circuit to the lens driving part of the optical pickup apparatus, changes the plurality of detection signals for tracking state detection with the adjusting circuit, and moves the objective lens by the prescribed signal based on the changed detection signals. The method further reciprocates the objective lens in the radial direction by prescribed distances for inspecting the state of deviation of the objective lens, obtains the current detection signals, and observes change of the detection signals. The adjusting circuit adjusts the detection signals to attain desired values. Thus, it is possible to correct deviation of the central portion of the objective lens by adjusting the adjusting circuit to change the detection signals in response to the amount of deviation of the mounting position of the objective lens with respect to the optical axes of the beams.
Further, the optical pickup apparatus itself is provided with the adjusting circuit, whereby the adjusting circuit can be adjusted in an assembling stage of the optical pickup apparatus. Thus, adjustment for correcting deviation of the central portion of the objective lens of the optical pickup apparatus can be omitted in the apparatus assembled with the optical pickup apparatus.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.